To understand the improvements provided by this invention, it will be helpful to understand that primary electrochemical cells are a class of voltaic cells, and that voltaic cells are those electrochemical cells in which chemical changes produce electrical energy. This distinguishes voltaic cells from electrolysis cells in which electrical energy from an outside source produces chemical changes within the cell. Primary cells cannot be conveniently recharged, and are discarded after a single exhaustion of their component elements since return to full power requires replacement of the exhausted chemical constituents. These cells are distinguished from another class of voltaic cells, namely, secondary cells, in which the exhausted cell is charged by passing electrical current from an outside source through it in the reverse direction. In the course of the voltaic cell reaction, current leaves the cathode and enters the anode, thus the cathode is the positive electrode and the anode is the negative electrode.
A particularly effective class of primary cells which employs soluble or liquid cathodes, as opposed to the more conventional solid cathode cells, has undergone rapid development in recent years. In these cells, the active cathode material is usually a fluid solvent for an electrolyte solute which provides conductivity. The active anode for these cells is usually lithium or other highly electropositive metal. During discharge, the solvent is electrochemically reduced on a cathode current collector to yield ions, e.g. halogen ions, which react with metallic ions from the anode to form soluble metal salts, e.g. metal halides. The cathode current collector does not take part in the reaction itself, but simply provides a support on which the reaction can occur, supplying electrons given up during the oxidation of the anode material.
The art has recognized that a wide variety of metals and semiconductor materials can be employed to make up the cathode current collector or to provide a catalytically active surface thereon. For example, in U.S. Pat. Nos. 3,926,669 and 4,012,564, eighteen different cathode current collectors are disclosed in the examples. The discharge current densities and open circuit potentials vary widely for these different materials, with gold being apparently a very desirable material. In U.S. Pat. No. 3,922,174, gold, carbon and (C.sub.4 F).sub.n are disclosed as catalytically active materials when used on the cathode current collector. In Example XI, a cathode current collector is disclosed having a gold plated titanium substrate.
Batteries for certain military applications, such as torpedo propulsion, must be capable of being discharged very rapidly, and provide large amounts of electrical energy within a relatively short period of time. The recently-developed liquid cathode cells of the type disclosed above have many significant advantages which make their use attractive for most applications where high performance is desired. To date, all such cells used in military and civilian applications have contained carbon black as the active catalyst on the cathode current collector; and, carbon black has demonstrated its ability to function in most situations. However, some cases demand greater current densities, without unduly increasing the cell volume. To meet this need, improved cathode current collectors must be developed.
Additional improvement could be obtained if there was some effective means for forming thin cathode current collectors of the more active catalyst materials, or for utilizing gold or other active catalyst materials in a more effective manner. Cathode current collectors have been made previously by extruding carbon black onto various substrates down to a thickness of 500 micrometers. However, it has been difficult to make thinner current collectors. Thus, the prior art has employed finely-divided carbon black in making cathode current collectors, and has, as previously noted, taught the use of solid gold for use on cathode current collectors; however, it does not suggest either the use of layers of finely-divided gold or other like catalysts other than carbon black, bonded to a less catalytically active substrate, or the use of layers of carbon black less than 500 micrometers thick bonded to a less catalytically active substrate.